Electrolyte health management for redox flow battery

ABSTRACT

Methods and systems are provided for a rebalancing reactor of a flow battery system. In one example, a pH of a battery electrolyte may be maintained by the rebalancing reactor by applying a negative potential to a catalyst bed of the rebalancing reactor. A performance of the rebalancing reactor may further be maintained by treating the catalyst bed with deionized water.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.16/684,429, entitled “ELECTROLYTE HEALTH MANAGEMENT FOR REDOX FLOWBATTERY”, and filed on Nov. 14, 2019. U.S. application Ser. No.16/684,429 claims priority to U.S. Provisional Application No.62/770,027, entitled “ELECTROLYTE HEALTH MANAGEMENT REDOX FLOW BATTERY”,and filed on Nov. 20, 2018. The entire contents of the above-listedapplications are hereby incorporated by reference for all purposes.

FIELD

The present description relates generally to a system for operating aredox flow battery system.

BACKGROUND AND SUMMARY

Performance degradation is a significant obstacle in battery systems,arising from a number of factors including, for example, side reactionsat the positive or negative electrode, internal shorting, ionicmovement, and catalyst poisoning. In a hybrid flow battery, such as aniron redox flow battery, undesirable reactions occurring in batteryelectrolyte may lead to capacity degradation, resulting in costlycompensation. For example, generation of hydrogen gas as well as ironcorrosion by proton (H⁺) and ferric (Fe³⁺) ions at the negativeelectrode may drive an electrolyte charge imbalance, thereby reducingbattery capacity. Furthermore, at least one side reaction describedabove may result in hydrogen evolution while iron corrosion occurring atthe negative electrode may cause electrolyte instability, furtherreducing a useful lifetime of a battery.

In order to maintain a performance of the battery, the electrolyte stateof charge (SOC) may be rebalanced by a supporting electrochemicalreaction occurring at an auxiliary rebalancing reactor. In one example,hydrogen gas produced at the negative electrode may be directed to acatalyst and contact between the hydrogen gas and the catalyst surfacemay chemically oxidize the hydrogen gas, returning protons to theelectrolyte. A low electrolyte pH for sustaining electrolyte stabilitymay be maintained as well as the balance of the electrolyte SOC.

However, the inventors herein have recognized potential issues with suchsystems. As one example, a presence of anions in the electrolyte mayinteract with the catalyst in a manner that degrades catalystperformance. The anions may adsorb onto the catalyst surface to form ananionic complex that induces formation of a cationic diffusion doublelayer. The positive double layer inhibits electro-active species fromreaching reaction sites on the catalyst, thus poisoning the catalyst andreducing an efficiency of the rebalancing reactor.

In one example, the issues described above may be addressed by a methodfor treating a rebalancing reactor of a flow battery including flowingan electrolyte of the flow battery and hydrogen gas generated in theflow battery to the rebalancing reactor, applying a negative potentialto a catalyst bed of the rebalancing reactor while charging the flowbattery, detecting a decrease in a ferric iron reduction rate at therebalancing reactor below a threshold rate, halting flow of electrolyteand hydrogen gas to the rebalancing reactor and then flowing deionizedwater through the rebalancing reactor in response to the decrease in theferric iron reduction, and indicating, after a threshold interval ofoperating time elapses, a request for soaking of the catalyst bed indeionized water. In this way, a likelihood of catalyst degradation isreduced and a performance of the flow battery is maintained.

As one example, the catalyst may be soaked in water at elevatedtemperatures to remove anionic complexes from the surface of thecatalyst in between operation of the rebalancing reactor. Fresh catalystsurface is exposed, enabling the catalyst to perform at a highercapacity. Additionally or alternatively, a negative potential may beapplied to the catalyst surface during operation of the rebalancingreactor, thereby repelling anions and reducing a likelihood of anionadsorption onto the catalyst.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic example of a flow battery system.

FIG. 2 shows a schematic example of a rebalancing reactor that may becoupled to the flow battery system.

FIG. 3A shows an example of a catalyst bed which may be used in arebalancing reactor, including sandwiched layers of a substrate, acatalyst, and a spacer.

FIG. 3B shows an end view of the catalyst bed rolled up into a jellyroll structure.

FIG. 3C shows a perspective side view of the catalyst bed rolled up intoa jelly roll structure.

FIG. 4 shows a first example of a reaction process occurring at arebalancing reactor catalyst.

FIG. 5 shows a second example of a reaction process occurring at arebalancing reactor catalyst.

FIG. 6 shows a graph illustrating an effect of ionic concentration on aperformance of the rebalancing reactor.

FIG. 7 shows a schematic diagram of an example of a testing apparatusfor evaluating a performance of the rebalancing reactor.

FIG. 8 shows a graph depicting datasets for a change in catalystperformance relative to a first variable of a first treatment process.

FIG. 9 shows a graph depicting datasets for a change in catalystperformance relative to a second variable of the first treatmentprocess.

FIG. 10 shows a graph depicting a comparison of catalyst performancewith and without a second treatment process applied during operation ofthe rebalancing reactor.

FIG. 11 shows an example of a method for maintaining catalystperformance in the rebalancing reactor by using a combination of thefirst and second treatment processes.

FIG. 12 is a continuation of the method of FIG. 11 .

FIG. 13 shows a graph depicting percent recovery of a catalyst bedversus an amount of deionized water used to soak the catalyst bed.

FIGS. 2-3C are shown approximately to scale.

DETAILED DESCRIPTION

The following description relates to systems and methods for arebalancing reactor for a redox flow battery. The redox flow battery maybe an all-iron flow battery (IFB) relying on iron redox reactions todrive a flow of electrons. An example of a redox flow battery system,which may include the IFB, is shown in a schematic diagram in FIG. 1 ,illustrating a plurality of battery components and an arrangement of theplurality of battery components in the IFB system. The plurality ofsystem components may include a rebalancing reactor to maintain a pH andstate of electrolyte charge in the IFB. For both a negative and apositive electrolyte of the IFB, the rebalancing reactor may beconfigured to oxidize hydrogen gas at a catalyst surface, the hydrogengas generated by a side reaction at a negative electrode of the IFB, asshown in a schematic representation in FIG. 2 . The catalyst may beincorporated in a catalyst bed, forming a layer sandwiched between asubstrate and a spacer, as shown in FIG. 3A. The catalyst bed may berolled up into a jelly roll structure to increase a packing density ofthe catalyst, the jelly roll arranging the catalyst in a spiralconfiguration as shown in FIGS. 3B-3C. Hydrogen oxidation may befacilitated at the catalyst surface on a carbon substrate of therebalancing reactor, shown in a first reaction scheme which may occur inthe rebalancing reactor in FIG. 4 . However, the hydrogen oxidationreaction may be impeded by formation of an anionic layer at thecatalytic reaction sites of the carbon substrate, as shown in FIG. 5 ina second reaction scheme which may occur at the carbon electrode. Anoverall concentration of ions in the electrolyte may degrade aperformance of the rebalancing reactor due the process shown in FIG. 5 .An effect of ionic concentration on a rate of iron reduction facilitatedby the catalyst of the rebalancing reactor is plotted in FIG. 6 . Bytreating the catalyst and a catalyst bed, the performance of therebalancing reactor may be improved. A first and a second treatmentprocess for the catalyst bed may be evaluated in a testing apparatusdepicted in a schematic diagram in FIG. 7 . A deterioration in catalyticperformance may be mitigated by a first treatment process that includessoaking the catalyst bed in water. Results of different catalyst bedsoaking times is shown in FIG. 8 in a graph plotting a rate of ironreduction in the rebalancing reactor with soaking time. A volume ofwater used to the soak the catalyst bed may also affect catalystperformance, as shown in a graph in FIG. 9 comparing an effect of watervolume on different types of catalysts. Degradation of catalystdurability may also be addressed by a second treatment process which mayinclude applying a potential to the catalyst bed (e.g., the catalystsupported on a substrate) during operation of the rebalancing system. Arate of iron reduction at the catalyst bed with an applied potential iscompared to a baseline rate of iron reduction in a graph depicted inFIG. 10 . Methods for maintaining the catalytic performance of therebalancing reactor is shown in FIGS. 11-12 . A method shown in FIG. 11is continued in FIG. 12 , the method incorporating a combination ofthree techniques, including applying a negative potential to thecatalyst bed, flushing the catalyst bed with DI water, and soaking thecatalyst bed in DI water to prolong a desired catalytic activity of therebalancing reactor. An effect of a volume of DI water used to soak adegraded catalyst bed is shown in FIG. 13 relative to a percent recoveryof catalyst performance.

FIGS. 1-5 show example configurations with relative positioning of thevarious components. If shown directly contacting each other, or directlycoupled, then such elements may be referred to as directly contacting ordirectly coupled, respectively, at least in one example. Similarly,elements shown contiguous or adjacent to one another may be contiguousor adjacent to each other, respectively, at least in one example. As anexample, components laying in face-sharing contact with each other maybe referred to as in face-sharing contact. As another example, elementspositioned apart from each other with only a space there-between and noother components may be referred to as such, in at least one example. Asyet another example, elements shown above/below one another, at oppositesides to one another, or to the left/right of one another may bereferred to as such, relative to one another. Further, as shown in thefigures, a topmost element or point of element may be referred to as a“top” of the component and a bottommost element or point of the elementmay be referred to as a “bottom” of the component, in at least oneexample. As used herein, top/bottom, upper/lower, above/below, may berelative to a vertical axis of the figures and used to describepositioning of elements of the figures relative to one another. As such,elements shown above other elements are positioned vertically above theother elements, in one example. As yet another example, shapes of theelements depicted within the figures may be referred to as having thoseshapes (e.g., such as being circular, straight, planar, curved, rounded,chamfered, angled, or the like). Further, elements shown intersectingone another may be referred to as intersecting elements or intersectingone another, in at least one example. Further still, an element shownwithin another element or shown outside of another element may bereferred as such, in one example.

Hybrid redox flow batteries are redox flow batteries that arecharacterized by deposition of one or more electro-active materials as asolid layer on an electrode. Hybrid redox flow batteries may, forinstance, include a chemical that plates via an electrochemical reactionas a solid onto a substrate throughout a battery charge process. Duringbattery discharge, the plated species may ionize also via anelectrochemical reaction, becoming soluble in the electrolyte. In hybridbattery systems, a charge capacity (e.g., amount of energy stored) ofthe redox battery may be limited by an amount of metal plated duringbattery charge and may depend on an efficiency of the plating system aswell as an available volume and surface area available for plating.

In a redox flow battery system a negative electrode, such as a negativeelectrode 26 of FIG. 1 , may be referred to as a plating electrode and apositive electrode, such as a positive electrode 28 of FIG. 1 , may bereferred to as a redox electrode. A negative electrolyte within aplating side (e.g., negative electrode compartment 20 of FIG. 1 ) of thebattery may be referred to as a plating electrolyte and a positiveelectrolyte on a redox side (e.g. positive electrode compartment 22 ofFIG. 1 ) of the battery may be referred to as a redox electrolyte.

Anode refers to an electrode where electro-active material loseselectrons and cathode refers to an electrode where electro-activematerial gains electrons. During battery charge, the positiveelectrolyte gains electrons at the negative electrode; therefore thenegative electrode is the cathode of the electrochemical reaction.During discharge, the positive electrolyte loses electrons; thereforethe negative electrode is the anode of the reaction. Accordingly, duringcharge, the negative electrolyte and negative electrode may berespectively referred to as a catholyte and cathode of theelectrochemical reaction, while the positive electrolyte and thepositive electrode may be respectively referred to as an anolyte andanode of the electrochemical reaction. Alternatively, during discharge,the negative electrolyte and negative electrode may be respectivelyreferred to as the anolyte and anode of the electrochemical reaction,while the positive electrolyte and the positive electrode may berespectively referred to as the catholyte and cathode of theelectrochemical reaction. For simplicity, the terms positive andnegative are used herein to refer to the electrodes, electrolytes, andelectrode compartments in redox battery flow systems.

One example of a hybrid redox flow battery is an all-iron redox flowbattery (IFB), in which the electrolyte comprises iron ions in the formof iron salts (e.g., FeCl₂, FeCl₃, and the like), wherein the negativeelectrode comprises metal iron. For example, at the negative electrode,ferrous ion, Fe²⁺, receives two electrons and plates as iron metal ontothe negative electrode during battery charge, and iron metal, Fe⁰, losestwo electrons and re-dissolves as Fe²⁺ during battery discharge. At thepositive electrode, Fe²⁺ loses an electron to form ferric ion, Fe³⁺,during charge, and during discharge Fe³⁺ gains an electron to form Fe²⁺.The electrochemical reaction is summarized in equations (1) and (2),wherein the forward reactions (left to right) indicate electrochemicalreactions during battery charge, while the reverse reactions (right toleft) indicate electrochemical reactions during battery discharge:Fe²⁺+2e ⁻↔Fe⁰ −0.44 V (Negative Electrode)  (1)2Fe²⁺↔2Fe³⁺+2e ⁻ +0.77 V (Positive Electrode)  (2)

As discussed above, the negative electrolyte used in the all-iron redoxflow battery (IFB) may provide a sufficient amount of Fe²⁺ so that,during charge, Fe²⁺ can accept two electrons from the negative electrodeto form Fe⁰ and plate onto a substrate. During discharge, the plated Fe⁰may then lose two electrons, ionizing into Fe²⁺ and dissolving back intothe electrolyte. The equilibrium potential of the above reaction is−0.44V and thus this reaction provides a negative terminal for thedesired system. On the positive side of the IFB, the electrolyte mayprovide Fe²⁺ during charge which loses electron and oxidizes to Fe₃₊.During discharge, Fe³⁺ provided by the electrolyte becomes Fe²⁺ byabsorbing an electron provided by the electrode. The equilibriumpotential of this reaction is +0.77V, creating a positive terminal forthe desired system.

The IFB provides the ability to charge and recharge its electrolytes incontrast to other battery types utilizing non-regenerating electrolytes.Charge is achieved by applying a current across the electrodes viaterminals, such as a negative terminal 40 and a positive terminal 42 inFIG. 1 . The negative electrode may be coupled via the negative terminalto a negative side of a voltage source so that electrons may bedelivered to the negative electrolyte via the positive electrode (e.g.,as Fe²⁺ is oxidized to Fe³⁺ in the positive electrolyte in the positiveelectrode compartment 22 in FIG. 1 ). The electrons provided to thenegative electrode (e.g., plating electrode) may reduce the Fe²⁺ in thenegative electrolyte to form Fe⁰ at the plating substrate causing it toplate onto the negative electrode.

Discharge may be sustained while Fe⁰ remains available to the negativeelectrolyte for oxidation and while Fe³⁺ remains available in thepositive electrolyte for reduction. As an example, Fe³⁺ availability maybe maintained by increasing a concentration or a volume of the positiveelectrolyte to the positive electrode compartment side of an IFB cell,such as cell 18 of FIG. 1 , to provide additional Fe³⁺ ions via anexternal source, such as an external positive electrolyte chamber orpositive electrolyte chamber. More commonly, availability of Fe⁰ duringdischarge may be an issue in IFB systems, wherein the Fe⁰ available fordischarge may be proportional to the surface area and volume of thenegative electrode substrate as well as the plating efficiency. Chargecapacity may be dependent on the availability of Fe²⁺ in the negativeelectrode compartment. As an example, Fe²⁺ availability can bemaintained by providing additional Fe²⁺ ions via an external source,such as an external negative electrolyte chamber to increase theconcentration or the volume of the negative electrolyte to the negativeelectrode compartment side of the IFB cell.

In an IFB, the positive electrolyte comprises ferrous ion, ferric ion,ferric complexes, or any combination thereof, while the negativeelectrolyte comprises ferrous ion or ferrous complexes, depending on thestate of charge of the IFB system. As previously mentioned, utilizationof iron ions in both the negative electrolyte and the positiveelectrolyte allows for utilization of the same electrolytic species onboth sides of the battery cell, which may reduce electrolytecross-contamination and may increase the efficiency of the IFB system,resulting in less electrolyte replacement as compared to other redoxflow battery systems.

Efficiency losses in an IFB may result from electrolyte crossoverthrough a separator, e.g., a separator 24 of FIG. 1 , (e.g.,ion-exchange membrane barrier, micro-porous membrane, and the like). Forexample, ferric ions in the positive electrolyte may be driven towardthe negative electrolyte by a ferric ion concentration gradient and anelectrophoretic force across the separator. Subsequently, ferric ionspenetrating the membrane barrier and crossing over to the negativeelectrode compartment may result in coulombic efficiency losses. Ferricions crossing over from the low pH redox side (e.g., more acidicpositive electrode compartment) to the high pH plating side (e.g., lessacidic negative electrode compartment) can result in precipitation ofFe(OH)₃. Precipitation of Fe(OH)₃ may degrade the separator and causepermanent battery performance and efficiency losses. For example,Fe(OH)₃ precipitate may chemically foul the organic functional group ofan ion-exchange membrane or physically clog the small micro-pores of anion-exchange membrane. In either case, due to the Fe(OH)₃ precipitate,membrane ohmic resistance may rise over time and battery performance maydegrade. Precipitate may be removed by washing the battery with acid,but the constant maintenance and downtime may be disadvantageous forcommercial battery applications. Furthermore, washing may be dependenton regular preparation of electrolyte, adding to process cost andcomplexity. Adding specific organic acids to the positive electrolyteand the negative electrolyte in response to electrolyte pH changes mayalso mitigate precipitate formation during battery charge and dischargecycling.

Additional coulombic efficiency losses may be caused by reduction of H⁺(e.g., protons) and subsequent formation of H₂ (e.g., hydrogen gas), andthe reaction of protons in the negative electrode compartment withelectrons supplied at the plated iron metal electrode to form hydrogengas.

The IFB electrolyte (e.g., FeCl₂, FeCl₃, FeSO₄, Fe₂(SO₄)₃, and the like)is readily available and may be produced at low costs. The IFBelectrolyte offers higher reclamation value because the same electrolytemay be used for the negative electrolyte and the positive electrolyte,consequently reducing cross contamination issues as compared to othersystems. Furthermore, owing to its electron configuration, iron maysolidify into a generally uniform solid structure during plating thereofon the negative electrode substrate. For zinc and other metals commonlyused in hybrid redox batteries, solid dendritic structures may formduring plating. The stable electrode morphology of the IFB system mayincrease the efficiency of the battery in comparison to other redox flowbatteries. Further still, iron redox flow batteries reduce the use oftoxic raw materials and can operate at a relatively neutral pH ascompared to other redox flow battery electrolytes. Accordingly, IFBsystems reduce environmental hazards as compared with all other currentadvanced redox flow battery systems in production.

During charge of an IFB, for example, ferrous ion, Fe²⁺, is reduced(accepts two electrons in a redox reaction) to metal iron, Fe⁰, at thenegative electrode. Simultaneously, at the positive electrode, ferrousion, Fe²⁺, is oxidized (loss of an electron) to ferric ion, Fe³⁺.Concurrently, at the negative electrode, the ferrous iron reductionreaction competes with the reduction of protons, H⁺, wherein two protonseach accept a single electron to form hydrogen gas, H₂ and the corrosionof iron metal to produce ferrous ion, Fe²⁺. The production of hydrogengas through reduction of hydrogen protons and the corrosion of ironmetal are shown in equations (3) and (4), respectively:H⁺ +e ⁻↔½H₂ (proton reduction)  (3)Fe⁰+2H⁺↔Fe²⁺+H₂ (iron corrosion)  (4)

As a result, the negative electrolyte in the negative electrodecompartment tends to stabilize at a pH range between 3 and 6. At thepositive electrode compartment, ferric ion, Fe³⁺, has a much lower aciddisassociation constant (pKa) than that of ferrous ion, Fe²⁺. Therefore,as more ferrous ions are oxidized to ferric ions, the positiveelectrolyte tends to stabilize at a pH less than 2, in particular at apH closer to 1.

Accordingly, maintaining the positive electrolyte pH in a first range inwhich the positive electrolyte (in the positive electrode compartment)remains stable and maintaining the negative electrolyte pH in a secondrange in which the negative electrolyte (in the negative electrodecompartment) remains stable may reduce low cycling performance andincrease efficiency of redox flow batteries. For example, maintaining apH of a negative electrolyte in an IFB between 3 and 4 may reduce ironcorrosion reactions and increase iron plating efficiency, whilemaintaining a pH of a positive electrolyte less than 2, and, inparticular, less than 1, may promote the ferric/ferrous ion redoxreaction and reduce ferric hydroxide formation.

As indicated by equation (3) and (4), evolution of hydrogen can causeelectrolyte imbalance in a redox flow battery system. For example,during charge, electrons flowing from the positive electrode to thenegative electrode (e.g., as a result of ferrous ion oxidation), may beconsumed by hydrogen evolution via equation (3), thereby reducing theelectrons available for plating given by equation (1). Because of thereduced plating, battery charge capacity is reduced. Additionally,corrosion of the iron metal further reduces battery capacity since adecreased amount of iron metal is available for battery discharge. Thus,an imbalanced electrolyte state of charge between the positive electrodecompartment and the negative electrode compartment can develop as aresult of hydrogen production via reaction (3) and (4). Furthermore,hydrogen gas production resulting from iron metal corrosion and protonreduction both consume protons, which can result in a pH increase of thenegative electrolyte. As discussed above, an increase in pH maydestabilize the electrolyte in the redox battery flow system, resultingin further battery capacity and efficiency losses.

An approach that addresses the electrolyte rebalancing issues that maybe caused by hydrogen gas production in redox flow battery systemscomprises reducing the imbalanced ion in the positive electrolyte withhydrogen generated from the side reactions. As an example, in an IFBsystem, the positive electrolyte comprising ferric ion may be reduced bythe hydrogen gas according to equation (5):Fe³⁺+½H₂→Fe²⁺+H⁺  (5)

In the IFB system example, by reacting ferric ion with hydrogen gas, thehydrogen gas can be converted back to protons, thereby maintain asubstantially constant pH in the negative electrode compartment and thepositive electrode compartment. Furthermore, by converting ferric ion toferrous ion, the state of charge of the positive electrolyte in thepositive electrode compartment may be rebalanced with the state ofcharge of the negative electrolyte in the negative electrodecompartment. Although equation (5) is written for rebalancingelectrolytes in an IFB system, the method of reducing an electrolytewith hydrogen gas may be generalized by equation (6):

$\begin{matrix}{{M^{x +} + {\frac{\left( {x - z} \right)}{2}H_{2}}}\rightarrow{M^{z +} + {\left( {x - z} \right)H^{+}}}} & (6)\end{matrix}$In equation (6), M^(x+) represents the positive electrolyte M havingionic charge, x, and M^(z+) represents the reduced electrolyte M havingionic charge, z.

A catalyst comprising graphite or comprising supported precious metal(e.g., carbon-supported Pt, Rd, Ru, or alloys thereof) catalyst mayincrease the rate of reaction described by equation (5) for practicalutilization in a redox flow battery system. As an example, hydrogen gasgenerated in the redox flow battery system may be directed to a catalystsurface, and hydrogen gas and electrolyte (e.g., comprising ferric ion)may be fluidly contacted at the catalyst surface, wherein the hydrogengas chemically reduces the ferric ion to ferrous ion and producespositive hydrogen ions (e.g., protons).

Returning to FIG. 1 , a schematic illustration of a generic redox flowbattery system 10 is depicted. In some examples, the redox flow batterysystem 10 may be the IFB system described above. The redox flow batterysystem 10 may comprise the redox flow battery cell 18, fluidly connectedto a multi-chambered electrolyte storage tank 110. The redox flowbattery cell 18 may generally include the negative electrode compartment20, the separator 24, and the positive electrode compartment 22. Theseparator 24 may comprise an electrically insulating ionic conductingbarrier which prevents bulk mixing of the positive electrolyte and thenegative electrolyte while allowing conductance of specific ionstherethrough. For example, the separator 24 may comprise an ion-exchangemembrane and/or a microporous membrane. The negative electrodecompartment 20 may comprise the negative electrode 26, and a negativeelectrolyte comprising electroactive materials. The positive electrodecompartment 22 may comprise the positive electrode 28, and a positiveelectrolyte comprising electroactive materials. In some examples,multiple redox flow battery cells 18 may be combined in series orparallel to generate a higher voltage or current in a redox flow batterysystem. Further illustrated in FIG. 1 are negative and positiveelectrolyte pumps 30 and 32, both used to pump electrolyte solutionthrough the flow battery system 10. Electrolytes are stored in one ormore tanks external to the cell, and are pumped via negative andpositive electrolyte pumps 30 and 32 through the negative electrodecompartment 20 side and the positive electrode compartment 22 side ofthe battery, respectively.

As illustrated in FIG. 1 , the redox flow battery cell 18 may furtherinclude the negative battery terminal 40, and the positive batteryterminal 42. When a charge current is applied to the battery terminals40 and 42, the positive electrolyte is oxidized (e.g., loses one or moreelectrons) at the positive electrode 28, and the negative electrolyte isreduced (e.g., gains one or more electrons) at the negative electrode26. During battery discharge, reverse redox reactions occur on theelectrodes. In other words, the positive electrolyte is reduced (e.g.,gains one or more electrons) at the positive electrode 28, and thenegative electrolyte is oxidized (e.g., loses one or more electrons) atthe negative electrode 26. The electrical potential difference acrossthe battery is maintained by the electrochemical redox reactions in thepositive electrode compartment 22 and the negative electrode compartment20, and may induce a current through a conductor while the reactions aresustained. An amount of energy stored by a redox battery is limited byan amount of electro-active material available in electrolytes fordischarge, depending on a total volume of electrolytes and a solubilityof the electro-active materials.

The flow battery system 10 may further comprise an integratedmulti-chambered electrolyte storage tank 110. The multi-chamberedstorage tank 110 may be divided by a bulkhead 98. The bulkhead 98 maycreate multiple chambers within the storage tank so that both thepositive and negative electrolyte may be included within a single tank.The negative electrolyte chamber 50 holds negative electrolytecomprising electroactive materials, and the positive electrolyte chamber52 holds positive electrolyte comprising electroactive materials. Thebulkhead 98 may be positioned within the multi-chambered storage tank110 to yield a desired volume ratio between the negative electrolytechamber 50 and the positive electrolyte chamber 52. In one example, thebulkhead 98 may be positioned to set the volume ratio of the negativeand positive electrolyte chambers according to the stoichiometric ratiobetween the negative and positive redox reactions. FIG. 1 furtherillustrates the fill height 112 of storage tank 110, which may indicatethe liquid level in each tank compartment.

FIG. 1 also shows gas head space 90 located above the fill height 112 ofnegative electrolyte chamber 50, and gas head space 92 located above thefill height 112 of positive electrolyte chamber 52. The gas head space92 may be utilized to store hydrogen gas generated through operation ofthe redox flow battery (e.g., due to proton reduction and corrosion sidereactions) and conveyed to the multi-chambered storage tank 110 withreturning electrolyte from the redox flow battery cell 18. The hydrogengas may be separated spontaneously at the gas-liquid interface (e.g.,fill height 112) within the multi-chambered storage tank 110, therebyprecluding having additional gas-liquid separators as part of the redoxflow battery system. Once separated from the electrolyte, the hydrogengas may fill the gas head spaces 90 and 92. As such, the stored hydrogengas can aid in purging other gases from the multi-chambered electrolytestorage tank 110, thereby acting as an inert gas blanket for reducingoxidation of electrolyte species, which can help to reduce redox flowbattery capacity losses. In this way, utilizing the integratedmulti-chambered storage tank 110 may forego having separate negative andpositive electrolyte storage tanks, hydrogen storage tanks, andgas-liquid separators common to conventional redox flow battery systems,thereby simplifying the system design, reducing the physical footprintof the system, and reducing system costs.

FIG. 1 also shows a spill-over hole 96, which creates an opening in thebulkhead 98 between gas head spaces 90 and 92, and provides a means ofequalizing gas pressure between the two chambers. The spill-over hole 96may be positioned at a threshold height above the fill height 112. Thespill-over hole 96 further enables a capability to self-balance theelectrolytes in each of the positive and negative electrolyte chambersin the event of a battery crossover. In the case of an all iron redoxflow battery system, the same electrolyte (Fe²⁺) is used in bothnegative and positive electrode compartments 20 and 22, so spilling overof electrolyte between the negative and positive electrolyte chambers 50and 52 may reduce overall system efficiency, but the overall electrolytecomposition, battery module performance, and battery module capacity aremaintained. Flange fittings may be utilized for all piping connectionsfor inlets and outlets to and from the multi-chambered storage tank 110to maintain a continuously pressurized state without leaks. Themulti-chambered storage tank can include at least one outlet from eachof the negative and positive electrolyte chambers, and at least oneinlet to each of the negative and positive electrolyte chambers.Furthermore, one or more outlet connections may be provided from the gashead spaces 90 and 92 for directing hydrogen gas to rebalancing reactors80 and 82.

Although not shown in FIG. 1 , integrated multi-chambered electrolytestorage tank 110 may further include one or more heaters thermallycoupled to each of the negative electrolyte chamber 50 and the positiveelectrolyte chamber 52. In alternate examples, only one of the negativeand positive electrolyte chambers may include one or more heaters. Inthe case where only the positive electrolyte chamber includes one ormore heaters, the negative electrolyte may be heated by transferringheat generated at the battery cells of the power module to the negativeelectrolyte. In this way, the battery cells of the power module may heatand facilitate temperature regulation of the negative electrolyte. Theone or more heaters may be actuated by a controller 88 to regulate atemperature of the negative electrolyte chamber 50 and the positiveelectrolyte chamber independently or together. For example, in responseto an electrolyte temperature decreasing below a threshold temperature,the controller may increase a power supplied to one or more heaters sothat a heat flux to the electrolyte is increased. The electrolytetemperature may be indicated by one or more temperature sensors mountedat the multi-chambered electrolyte storage tank 110, including sensors60 and 62.

As examples, the one or more heaters may include coil type heaters orother immersion heaters immersed in the electrolyte fluid, or surfacemantle type heaters that transfer heat conductively through the walls ofthe negative and positive electrolyte chambers to heat the fluidtherein. Other known types of tank heaters may be employed withoutdeparting from the scope of the present disclosure. Furthermore,controller 88 may deactivate one or more heaters in the negative andpositive electrolyte chambers in response to a liquid level decreasingbelow a solids fill threshold level. Said in another way, controller 88may activate the one or more heaters in the negative and positiveelectrolyte chambers only in response to a liquid level increasing abovethe solids fill threshold level. In this way, activating the one or moreheaters without sufficient liquid in the positive and/or negativeelectrolyte chambers can be averted, thereby reducing a risk ofoverheating or burning out the heaters.

Further illustrated in FIG. 1 , electrolyte solutions typically storedin the multi-chambered storage tank 110 are pumped via negative andpositive electrolyte pumps 30 and 32 throughout the flow battery system10. Electrolyte stored in negative electrolyte chamber 50 is pumped vianegative electrolyte pump 30 through the negative electrode compartment20 side, and electrolyte stored in positive electrolyte chamber 52 ispumped via positive electrolyte pump 32 through the positive electrodecompartment 22 side of the battery.

Two electrolyte rebalancing reactors 80 and 82, components of arebalancing system 85 of the IFB, may be connected in-line or inparallel with the recirculating flow paths of the electrolyte at thenegative and positive sides of the battery, respectively, in the redoxflow battery system 10. The rebalancing system 85 may include one ormore rebalancing reactors connected in-line with the recirculating flowpaths of the electrolyte at the negative and positive sides of thebattery, and other rebalancing reactors may be connected in parallel,for redundancy (e.g., a rebalancing reactor may be serviced withoutdisrupting battery and rebalancing operations) and for increasedrebalancing capacity.

In one example, the electrolyte rebalancing reactors 80 and 82 may beplaced in the return flow path from the positive and negative electrodecompartments 20 and 22 to the positive and negative electrolyte chambers50 and 52, respectively. The rebalancing system 85 may serve torebalance electrolyte charge imbalances in the redox flow battery system10 occurring due to side reactions, ion crossover, and the like, asdescribed herein. In one example, electrolyte rebalancing reactors 80and 82 may include trickle bed reactors, where the hydrogen gas andelectrolyte are contacted at catalyst surfaces in a packed bed forcarrying out the electrolyte rebalancing reaction. Alternatively, therebalancing reactors 80 and 82 may have catalyst beds configured as ajelly roll, in shown in FIGS. 3A-3C and discussed further below.Furthermore, methods for maintaining an activity of the catalystsurfaces are provided further below with respect to FIGS. 7-13 . Inother examples the rebalancing reactors 80 and 82 may includeflow-through type reactors that are capable of contacting the hydrogengas and the electrolyte liquid and carrying out the rebalancingreactions in the absence of a packed catalyst bed.

During operation of a redox flow battery system, sensors and probes maymonitor and control chemical properties of the electrolyte such aselectrolyte pH, concentration, state of charge, and the like. Forexample, as illustrated in FIG. 1 , sensors 62 and 60 may be positionedto monitor positive electrolyte and negative electrolyte conditions atthe positive electrolyte chamber 52 and the negative electrolyte chamber50, respectively. As another example, sensors 72 and 70, alsoillustrated in FIG. 1 , may monitor positive electrolyte and negativeelectrolyte conditions at the positive electrode compartment 22 and thenegative electrode compartment 20, respectively. In one example, sensors70 and 72 may include pressure sensors that transmit signals to thecontroller 88 indicating the pressure at the negative and positive sidesof the separator 24 of the redox flow battery cell 18. The pressure atthe negative and positive electrode compartments 20 and 22 of theseparator 24 may be regulated by controlling the inlet and outlet flowsof negative and positive electrolyte thereto, respectively. For example,the controller may decrease a pressure at the negative electrodecompartment 20 by one or more of increasing a pump speed of a vacuumpump fluidly coupled to thereto, reducing a pump speed of the negativeelectrolyte pump 30, and by throttling a back pressure flow regulator toincrease an outlet flow from the negative electrode compartment.

Similarly, the controller may increase a pressure at the positiveelectrode compartment 22 by one or more of increasing a pump speed ofthe positive electrolyte pump 32, and by throttling a back pressure flowregulator to decrease an outlet flow from the negative electrodecompartment. Back pressure flow regulators may include orifices, valves,and the like. For example, controller 88 may send a signal to position avalve to a more open position, to induce higher outlet flows fromnegative electrode compartment 20, thereby reducing a negative electrodecompartment pressure. Increasing the positive electrode compartmentpressure and decreasing the pressure in the negative electrodecompartment may aid in increasing a cross-over pressure (positive overnegative) across the separator 24. Increasing the cross-over pressure byincreasing the flow of the positive electrolyte by increasing the pumpspeed of the positive electrolyte pump 32 and increasing back pressureat the outlet of the positive electrode compartment 22 may be lessdesirable than other methods of increasing the cross-over pressurebecause pump parasitic losses may be increased.

Sensors may be positioned at other locations throughout the redox flowbattery system to monitor electrolyte chemical properties and otherproperties, such as temperature, fluid pressure, current, voltage etc.For example, a sensor may be positioned in an external acid tank (notshown) to monitor acid volume or pH of the external acid tank, whereinacid from the external acid tank is supplied via an external pump (notshown) to the redox flow battery system 10 in order to reduceprecipitate formation in the electrolytes. Additional external tanks andsensors may be installed for supplying other additives to the redox flowbattery system 10. For example, various sensors including, temperature,pressure, conductivity, and level sensors of a field hydration systemmay transmit signals to the controller 88 when hydrating the redox flowbattery system 10 in a dry state. Furthermore, controller 88 may sendsignals to actuators such as valves and pumps of the field hydrationsystem during hydration of the redox flow battery system 10. Sensorinformation may be transmitted to controller 88 which may in turnactuate negative and positive electrolyte pumps 30 and 32 to controlelectrolyte flow through the cell 18, or to perform other controlfunctions, as an example. In this manner, the controller 88 may beresponsive to, one or a combination of sensors and probes. Redox flowbattery cell 18 may be positioned within one of a plurality of redoxflow battery cell stacks of a power module for a redox flow batterysystem. Each of the redox flow battery cells 18 in a redox flow batterycell stack may be electrically connected in series and/or parallel witha plurality of other redox flow battery cells in the redox flow batterycell stack. Furthermore, each of the redox flow battery cell stacks maybe electrically connected in series and/or parallel with a plurality ofthe other redox flow battery cell stacks in the power module. In thisway, the redox flow battery cell stacks may be electrically combined tosupply power from the power module.

Redox flow battery system 10 may further comprise a source of hydrogengas. In one example the source of hydrogen gas may comprise a separatededicated hydrogen gas storage tank. In the example of FIG. 1 , hydrogengas may be stored in and supplied from the integrated multi-chamberedelectrolyte storage tank 110. Integrated multi-chambered electrolytestorage tank 110 may supply additional hydrogen gas to the positiveelectrolyte chamber 52 and the negative electrolyte chamber 50.Integrated multi-chambered electrolyte storage tank 110 may alternatelysupply additional hydrogen gas to the inlet of electrolyte rebalancingreactors 80 and 82. As an example, a mass flow meter or other flowcontrolling device (which may be controlled by controller 88) mayregulate the flow of the hydrogen gas from integrated multi-chamberedelectrolyte storage tank 110.

The integrated multi-chambered electrolyte storage tank 110 maysupplement the hydrogen gas generated in the redox flow battery system10. For example, when gas leaks are detected in the redox flow batterysystem 10 or when the reduction reaction rate is too low at low hydrogenpartial pressure, hydrogen gas may be supplied from the integratedmulti-chambered electrolyte storage tank 110 in order to rebalance thestate of charge of the electro-active species in the positiveelectrolyte and negative electrolyte. As an example, controller 88 maysupply hydrogen gas from integrated multi-chambered electrolyte storagetank 110 in response to a measured change in pH or in response to ameasured change in state of charge of an electrolyte or anelectro-active species. For example an increase in pH of the negativeelectrolyte chamber 50, or the negative electrode compartment 20, mayindicate that hydrogen is leaking from the redox flow battery system 10and/or that the reaction rate is too slow with the available hydrogenpartial pressure. In response to the pH increase, controller 88 mayincrease a supply of hydrogen gas from integrated multi-chamberedelectrolyte storage tank 110 to the redox flow battery system 10. As afurther example, controller 88 may supply hydrogen gas from integratedmulti-chambered electrolyte storage tank 110 in response to a pH change,wherein the pH increases beyond a first threshold pH or decreases beyonda second threshold pH.

In the case of an IFB, controller 88 may supply additional hydrogen toincrease the rate of reduction of ferric ions and the rate of productionof protons, thereby reducing the pH of the positive electrolyte.Furthermore, the negative electrolyte pH may be lowered by hydrogenreduction of ferric ions crossing over from the positive electrolyte tothe negative electrolyte or by protons generated at the positive sidecrossing over to the negative electrolyte due to a proton concentrationgradient and electrophoretic forces. In this manner, the pH of thenegative electrolyte may be maintained within a stable region, whilereducing the risk of precipitation of ferric ions (crossing over fromthe positive electrode compartment) as Fe(OH)₃.

Other control schemes for controlling the supply rate of hydrogen gasfrom integrated multi-chambered electrolyte storage tank 110 responsiveto a change in an electrolyte pH or to a change in an electrolyte stateof charge, detected by other sensors such as an oxygen-reductionpotential (ORP) meter or an optical sensor, may be implemented. Furtherstill, the change in pH or state of charge triggering the action ofcontroller 88 may be based on a rate of change or a change measured overa time period. The time period for the rate of change may bepredetermined or adjusted based on the time constants for the redox flowbattery system 10. For example, the time period may be reduced if therecirculation rate is high, and local changes in concentration (e.g.,due to side reactions or gas leaks) may quickly be measured since thetime constants may be small. As described above, a rebalancing system,such as the rebalancing system 85 of FIG. 1 , may include one or morerebalancing reactors, e.g., the rebalancing reactors 80 and 82 of FIG. 1, forming an electrochemical cell used to maintain the pH and stabilityof electrolytes for an IFB battery. Reactions occurring at therebalancing reactors are described below by equations (7) and (8):H⁺ +e ⁻↔½H₂ (Hydrogen proton reduction)  (7)Fe⁰+2H⁺↔Fe²⁺+H₂ (Iron corrosion)  (8)

The electrochemical cell may also be referred to as a rebalancing cellthat includes an anode, which may be the rebalancing reactor 80 of FIG.1 , and a cathode, which may be the rebalancing reactor 82 of FIG. 1 .

A schematic diagram of a rebalancing cell 200 is depicted in FIG. 2 .Hydrogen (H₂), evolved from the negative electrode of the IFB battery,flows through a flow field to an anodic side 210 of the rebalancing cell200. Therein, electrons are stripped from the hydrogen molecules by ahydrogen oxidation catalyst 202, thereby producing protons andelectrons, as indicated by reaction 211. The hydrogen oxidation catalyst202 may comprise a small amount (e.g. 0.02 mg/cm² to >0.2 mg/cm²) ofprecious metal, such as Pt, Pd, Ru, Rd or their alloys, supported on aconductive carrier, such as carbon. IFB electrolytes containing ferricions (Fe³⁺) flow through a cathodic side 220 of the rebalancing cell200, arranged on an opposite side of a membrane separator 206 from theanodic side 210. On the cathodic side 220, electrons are accepted byferric ions on a carbon electrode 204, thereby producing ferrous ions,as indicated by reaction 221. The hydrogen oxidation catalyst 202,acting as an anode, may be separated from the carbon electrode 204,acting as a cathode, by the membrane separator 206 which allowsselective exchange of ions between the anodic side 210 and the cathodicside 220.

The rebalancing cell 200 may include a conductive wire 230 coupled to acatalyst bed 208 in which the hydrogen oxidation catalyst 202 isembedded. The conductive wire 230 may be formed of a metal such astitanium and may be inserted into the catalyst bed so that the wire issecured within the catalyst bed. The conductive wire 230 may be woven,for example, in a sinuous pattern to maximize a coverage of theconductive wire 230 across a surface area of the catalyst bed 208. Avoltage is conducted by the conductive wire 230 from an electricalenergy storage device 232, such as a battery. Further details of a useof the conductive wire 230 to apply a potential to a surface of thehydrogen oxidation catalyst 202 are described below with reference toFIGS. 10-11 .

The resulting Gibbs free energy of reactions 211 and 221 is negative.The reactions therefore occur spontaneously, resulting in the hightheoretically electrical efficiency. The energy released from theseelectrochemical cells may be used to power auxiliary components in theoverall IFB system, for example, electronic components, cooling fans,and/or indication lights, thus improving overall system efficiency. Theenergy, i.e. voltage, generated through the application of an electricalload to the electrochemical cell may be stored in a system power bus.

A rebalancing cell, e.g., the rebalancing cell 200 of FIG. 2 , may beincluded in a rebalancing reactor, such as the rebalancing reactors 80and 82 of FIG. 1 , to offset a loss of protons due to equation (3) andstabilize an electrolyte pH, thereby also suppressing the iron corrosionreaction of equation (4). In one example, the rebalancing cell may beadapted as a packed catalyst bed housing within a trickle bed reactorand hydrogen may be directed from gas head spaces of electrolyte storagechambers with respect to the gas head spaces 90 and 92 of the negativeelectrolyte chamber 50 and positive electrolyte chambers 52,respectively, of FIG. 1 , to one or more trickled bed reactors coupledto the electrolyte storage tanks. In another example, as shown in FIGS.3A-3C, the rebalancing cell may be configured in a jelly roll structure,which may increase an efficiency of the rebalancing reactor towardoxidizing hydrogen gas.

In FIGS. 3A-3C, an example of a catalyst bed 300 is shown. As oneexample, the catalyst bed 300 may include the rebalancing cell 200 shownin FIG. 2 . A section of a single layer of the catalyst bed 300 is shownfrom a profile view in FIG. 3A, arranged in a planar orientation forclarity. The catalyst bed 300 may be formed by coating a substrate layer304 with a catalyst layer 306. One or both sides of the substrate layer304 may be coated with the catalyst layer 306. Coating both sides of thesubstrate layer 304 may increase a redox reaction rate of the catalystbed 300 as compared to coating a single side of the substrate layer 304.

Substrate layer 304 may include a flexible and bendable substrate suchas carbon cloth, carbon paper, or another type of membrane. Substratelayer 304 may be porous or non-porous, and/or permeable to hydrogen gas,hydrogen ions, and to electrolyte, such as positive electrolyte andnegative electrolyte from positive electrolyte chamber 52 and negativeelectrolyte chamber 50 of FIG. 1 . Substrate layer 304 may further beinert with respect to hydrogen gas, hydrogen ions, and the electrolyteincluding both the positive electrolyte and the negative electrolyte. Athickness 308 of the substrate layer 304 may be small enough so as notto substantially hinder diffusion or convective transport of electrolytespecies through the substrate layer 304. For example, when the substratelayer 304 is thinner than 0.5 mm, reaction rates may be higher ascompared to when the substrate layer 304 is thicker than 0.5 mm.

The substrate layer 304 may be conductive, semi-conductive, ornon-conductive. Conductive substrate layers may yield higher reactionrates as compared to non-conductive substrate layers. For example, acarbon substrate (e.g., carbon cloth, carbon paper, and the like) mayaid in electron transfer, and provides a catalytic surface for theferric/ferrous ion redox reaction. Some example membrane materials thatmay be utilized for the substrate layer 304 include polypropylene,polyolefin, perfluoroalkoxy (PFA), polysulfone amide (PSA), and thelike. In addition, the substrate layer 304 may comprise a thin ceramicsheet or a thin metal sheet, provided the substrate layer 304 does notreact with ferric ions.

Catalyst layer 306 may include one or more different types of catalystmaterials such as platinum, palladium, ruthenium, alloys thereof. Theweight percent of the catalyst material on the substrate layer 304 maybe from 0.2 wt % to greater than 0.5 wt %. The substrate layer 304coated with the catalyst layer 306 may be porous and permeable tohydrogen gas, hydrogen ions, and to electrolyte including the positiveelectrolyte and the negative electrolyte. When hydrogen gas and metalions in the electrolyte are fluidly contacted at the catalyst layer 306,the catalyst layer 306 may catalyze a redox reaction whereby thehydrogen gas may be oxidized to hydrogen ions and the metal ions may bereduced (e.g. reactions 221 and 221 of FIG. 2 ). The substrate layer 304may be coated entirely with the catalyst layer 306 to increase a redoxreaction rate of hydrogen gas and metal ions at the catalyst layersurface.

Catalyst bed 300 may further comprise a spacing layer 310 positioned onthe catalyst layer. As shown in FIGS. 3A-3C, the spacing layer 310 maybe thinner than the substrate layer 304, however in other examples, thesubstrate layer 304 may be thinner than the spacing layer 310. Thinnerspacing layers may yield higher catalyst bed reaction rates with higherpressure drops across the catalyst bed while thicker spacing layers mayyield lower reaction rates with lower pressure drops across the catalystbed. In some examples the spacing layer 310 may be less than 1 mm thick.The spacing layer 310 may comprise a mesh, such as a plastic or othertype of non-conductive mesh. For example, the spacing layer may comprisea polypropylene, polyolefin, polyethylene, polystyrene, or other polymermesh that is stable (e.g., does not react with or degrade in thepresence of) ferric/ferrous ion solutions. In other examples, thespacing layer may comprise and open-celled plastic foam or spongematerial.

A conductive wire 330, which may be similar to the conductive wire 230of FIG. 2 , may be woven through the catalyst layer 306 so that theconductive wire 330 is in close proximity to the catalyst material,e.g., in contact with or near catalyst sites. The conductive wire 300may have a linear, sinuous, zig zag, etc. layout across the z-x plane inthe catalyst layer 306 and extend out of the catalyst bed 300 to coupleto an electrical energy storage device 332, hereafter battery 332. Avoltage supplied by the battery 332 may be conducted to the catalystlayer 306 via the conductive wire 330.

The catalyst bed 300 may be spiral wound to form a jelly roll structuredcatalyst bed 320, as depicted in FIGS. 3B-3C. A set of reference axes301 are provided for comparison between views, indicating a y-axis, anx-axis, and a z-axis. As shown in FIG. 3B in a view of an end of thejelly roll structured catalyst bed 320, along an x-z plane, eachsuccessive substrate layer 304 and catalyst layer 306 of the spiralwound jelly roll structured catalyst bed 320 is separated by the spacinglayer 310. The spacing layer 310 may entirely cover the catalyst layer306. In this way, each catalyst layer 306 is entirely separated from anadjacent catalyst layer by the spacing layer 310 when the substratelayer 304 is coated on both sides by the catalyst layer 306. As shown ina perspective side view of the jelly roll structured catalyst bed inFIG. 3C, the spacing layers 310 may extend across the entire axialdimension, e.g., along the y-axis, of the jelly roll structured catalystbed 320, as indicated by dashed lines.

When coiled into the jelly roll structure as shown in FIG. 3C, the jellyroll structured catalyst bed 320 has a cylindrical shape. Thecylindrical, rolled configuration of the jelly roll structured catalystbed 320 may allow the jelly roll structured catalyst bed 320 to beremoved as a single unit, reducing time and costs of maintenance of arebalancing reactor. Electrolyte may be flowed through the jelly rollstructured catalyst bed 320, the flow in contact with the catalyst bed320 for a prolonged period of time in comparison to packed catalystbeds, increasing an efficiency of the jelly roll structured catalyst bed320 in facilitating hydrogen oxidation and iron reduction.

The conductive wire 330 may be incorporated into the catalyst layer 306so that an end of the conductive wire 330 that ends of the conductivewire 330 that couple directly to the battery 332 extend out of the jellyroll structured catalyst bed 320 in an axial direction, along the y-axisas shown in FIG. 3C. The jelly roll structure catalyst bed 320 may beinserted into an outer housing that is also cylindrical to match a shapeof the jelly roll structure catalyst bed 320, sliding in and out of thehousing along a central axis of rotation of the cylindrical outerhousing. Extension of the conductive wire 330 from a top or a bottom,with respect to the y-axis, of the jelly roll structure catalyst bed 320allows the conductive wire 330 to be readily connected to the battery332 through a top or a bottom of the outer housing of the jelly rollstructured catalyst bed 320.

As described above, a catalyst of a catalyst bed used in a rebalancingreactor may be a metal, such as platinum (Pt). A single catalyst site ofthe catalyst layer 306 is depicted in FIGS. 4 and 5 for brevity but itwill be appreciated that catalyst bed may include a plurality ofcatalyst sites. As shown in a first scheme 400 in FIG. 4 , a Pt site 402catalyzes a transfer of an electron from half of a hydrogen gas moleculeat a carbon substrate 404 to oxidize the hydrogen gas to a proton. Thefirst reaction scheme 400 may occur at Pt sites along the catalyst layer306 of the jelly roll structured catalyst bed 320 of FIGS. 3B-3C. Theelectron obtained from the hydrogen molecule is used to reduce ferriciron to ferrous iron at the carbon substrate, e.g. the substrate layer304 of FIGS. 3A-3C, in contact with electrolyte containing iron cations.

The first scheme 400 may represent an intended process for which therebalancing reactor is configured to perform when the electrolytecomprises exclusively redox active species such as ferric and ferrousiron complexes. However the electrolyte may include supporting,non-redox active species such as potassium cations and chloride anionsfrom dissolved potassium chloride salt. The supporting electrolytespecies may increase an ionic strength and hence a conductivity of theelectrolyte. A presence of the supporting species, however, may degradean activity of the Pt sites.

A second scheme 500 shown in FIG. 5 may occur in the rebalancing reactorwhen supporting electrolyte species are present, competing with andinhibiting or partially-inhibiting implementation of the first scheme400. In the second scheme 500, chloride anions in the electrolyte mayadsorb onto a Pt site 502, supported on a carbon substrate 504. Enoughchloride anions may surround the Pt site to form a negatively chargedlayer around the Pt site. The negatively charged layer may attractcations 506, e.g., potassium cations, in the electrolyte, creating apositively charged layer around the negatively charged layer. Together,the adsorbed chloride anions and the surrounding potassium cations mayform a surface charge layer and a diffuse layer, respectively, resultingin a charge double diffusion layer that inhibits or partially inhibitsinteraction between the intended reactants of the rebalancing reaction,such as between hydrogen gas and iron cations, and the Pt site. Apresence of the charge double diffusion layer may thus degrade aperformance of the catalyst bed by at least partially inhibiting contactbetween the intended reactants of the rebalancing reactor and thecatalyst.

A performance of a catalyst bed of a rebalancing reactor may be directlyaffected by a concentration of supporting ionic species in anelectrolyte. When a concentration of the supporting ions is lower (e.g.,below a threshold concentration), the diffuse positively charged layerof the charge double diffusion layer around the Pt sites may beprimarily formed from ferric and ferrous ions, allowing the iron cationsto diffuse through the relatively sparse chloride surface charge layerto the Pt site. Increasing the ionic concentration, however, can resultin an increase in supporting cations (e.g., potassium) that compete withthe iron cations to form the double diffusion layer. When the doublediffusion layer is formed predominantly by potassium cations, the ferricand ferrous ions are blocked from the Pt site by a boundary layer ofpositively charged potassium as well as a dense layer of chlorideanions. The double diffusion layer also impedes diffusion of hydrogengas molecules to the Pt site, thus hindering both an oxidation ofhydrogen gas and a reduction of ferric iron.

An effect of ionic concentration on the rebalancing reactor is shown ina graph 600 in FIG. 6 . A rate of iron reduction from ferric to ferrousiron, which may be a measure of the performance of the rebalancingreactor, is plotted along the y-axis of graph 600 and total ionicconcentration of the electrolyte in moles/liter (M) is plotted along thex-axis. A first plot 602 of graph 600 depicts a change in reduction ratewith ionic concentration for a first Pt catalyst and a second plot 604depicts a change in reduction rate with ionic concentration for a secondPt catalyst. The first Pt catalyst and the second Pt catalyst may besimilarly used and differentiated by different carbon substrates. Thesecond Pt catalyst may be supported on a more hydrophobic carbon supportthan the first Pt catalyst. Both the first and second plots 602, 604show a decrease in ferric reduction rate as ionic concentrationincreases. The dependence on the total ionic concentration for is higherfor the second Pt catalyst supported on the more hydrophobic carbonsupport.

Performance degradation of a catalyst bed in a rebalancing reactor of anIFB caused by formation of the charged diffusion double layer may bemitigated by treating the catalyst bed in a manner that counteractsadsorption of the anions at the catalyst surface. For example, suchtreatment may include removing adsorbed anions from catalyst sitesand/or repelling anions from the catalyst sites. Effects of variouscatalyst bed treatments for mitigating catalyst bed degradation may beevaluated using a testing apparatus 700, as shown in a schematic diagramin FIG. 7 .

The testing apparatus 700 includes an electrolyte container 702, storingelectrolyte used by an IFB. The electrolyte may flow in a directionindicated by arrows 704, from the electrolyte container 702, through aperistaltic pump 706. Activation of the peristaltic pump 706 drives amovement of the electrolyte, compelling electrolyte to flow through anelectrolyte circuit 708 of the testing apparatus 700. The electrolyte ispumped to an injector 710 that is coupled to a high pressure hydrogenbottle 712 containing pressurized hydrogen gas. A valve 714 ispositioned between the hydrogen bottle and the injector, controllingflow of hydrogen gas to the injector. When the valve is opened, hydrogengas is injected into the electrolyte at the injector 710. The hydrogeninfused electrolyte is flowed to a rebalancing reactor 716.

In one example, the rebalancing reactor 716 may be configured with ajelly roll structured catalyst bed 718. In other examples, therebalancing reactor 702 may have a packed catalyst or be adapted withsome other catalyst configuration. The jelly roll structured catalystbed 718 may be a coiled sheet of a carbon supported Pt catalyst on acarbon substrate layer, such as the jelly roll structured catalyst bed320 shown in FIGS. 3B and 3C. Electrolyte may flow in a directionindicated by arrows 704 through the electrolyte circuit 708 to return tothe electrolyte container 702. An ionic concentration of the electrolytemay be adjusted by varying a concentration of potassium chloride (KCl)in the electrolyte.

Using the testing apparatus 700 of FIG. 7 , effects of a first treatmentapplied to the rebalancing reactor 716 may be evaluated. It will beappreciated that the testing apparatus 700 of FIG. 7 is a non-limitingexample and other systems may be similarly used to evaluate therebalancing reactor. The first treatment may include soaking the jellyroll structured catalyst bed 718 (hereafter, catalyst bed 718) of therebalancing reactor 716 in deionized (DI) water with the catalyst bed718 unrolled. The catalyst bed 718 may be removed from an outer housingof the rebalancing reactor 716 and placed in a sealed container filledwith a known amount of DI water. The sealed container may be placed inan oven and heated to a target temperature, such as 50° C., for a periodof time. In other examples, the target temperature may be 70° C. or 80°C. or any temperature between room temperature and 100° C. A set ofvariables including the oven temperature, a volume of DI water, and theamount of time the catalyst bed 718 is heated in the oven may beindependently varied to separate effects of each variable.

The catalyst bed 718 is returned to the outer housing of the rebalancingreactor 716 after removal from the DI water, rinsing with fresh DIwater, and re-coiled into the jelly roll structure. The peristaltic pump706 of the testing apparatus 700 may be activated to pump electrolytethrough the electrolyte circuit 708. As the electrolyte flows throughthe rebalancing reactor 716, a rate of ferric iron reduction may bedetermined by measuring a change in ferric iron concentration in theelectrolyte by cyclic voltammetry, redox titration, or oxygen reductionpotential measurements.

An effect of soaking time and temperature of a catalyst bed, e.g., thecatalyst bed 718 of FIG. 7 on ferric iron reduction is shown in a graph800 in FIG. 8 . A rate of ferric ion reduction, in moles per squaremeter hour, is measured along the y-axis and a time of soaking in DIwater is shown along the x-axis of graph 800. Two sets of data are shownin graph 800: a first data set 802 (circles) representing soaking of thecatalyst bed at 80° C. and a second data set 804 (diamonds) representingsoaking of the catalyst bed at 90° C. An initial rate of ferric ironreduction (not shown in graph 800) may be, for example, 0.73 mol/m² hrafter numerous cycles of usage of the catalyst bed in a rebalancingreactor.

The first data set 802 includes two data points at, for example, 18hours and 78 hours of catalyst bed soaking in DI water. The longersoaking time corresponds to a higher reduction rate by 0.2 mol/m² hr.The second data set 804 includes three data points at, for example, 89,125, and 188 hours of catalyst bed soaking, all three data pointscorresponding to higher reduction rates than the rates for the firstdata set 802. The second data set 804 also shows an increase in the rateof ferric iron reduction with longer soaking times. The increase inreduction rate between 125 and 188 hours is less than then the increasein reduction rate between 89 and 125 hours as well as less than thenincrease in reduction rate shown by the first data set 802.

The first data set 802 and second data set 804 displayed in graph 800indicate that soaking the catalyst bed in warmer DI water increases therate of ferric iron reduction. Longer soaking times also result inincreased reduction rates although an effect of soaking period may taperas soaking time increase beyond 188 hours. Soaking the catalyst bed inwarm DI water may effect removal of adsorbed chloride anions from acatalyst surface. The negatively charged chloride anions may experiencevan der Waals forces attracting the chloride anions to dipolar watermolecules. Over time, the van der Waals forces may overcome adsorptionof the chloride anions to the catalyst surface, thus stripping thecatalyst surface of a negatively charged surface layer. In other words,maintaining a catalyst bed temperature above a threshold DI soakingtemperature during DI soaking of the catalyst bed may aid in increasingthe recovered performance of a degraded catalyst bed relative to lowercatalyst bed temperatures. Furthermore, soaking a catalyst bed in DIwater longer than a threshold amount of time may aid in increasing therecovered performance of a degraded catalyst bed relative to shorter DIsoaking times. An increased recovered performance may refer to higherrebalancing reaction rates (e.g., ferric reduction rates for catalystbeds in IFB rebalancing reactors). For example, the threshold DI soakingtemperature may be greater than 80° C.; in another example the thresholdDI soaking temperature may be greater than 90° C. In one example, thethreshold DI soaking time may be greater than 60 hours.

A number of water molecules present to interact with the catalyst bedmay also affect a change in ferric iron reduction rate after soaking inDI water. For example, a volume of DI water used to soak the catalystbed may determine an amount of chloride anions removed from the catalystsurface. An effect of DI water volume on a rate of ferric iron reductionis depicted in a graph 900 in FIG. 9 . A testing apparatus, such as thetesting apparatus 700 of FIG. 7 may be used to evaluate an influence ofvolume on a jelly roll structured catalyst bed. Data sets showing rateof ferric ion reduction, in moles per square meter hour, are displayedin columns, representing five different catalysts used in the catalystbed.

A first data set 902 of graph 900 shows baseline reduction rates foreach type of catalyst, e.g., reduction rates without soaking in DIwater. The baseline reduction rates vary slightly, ranging, for example,between 0.9 and 1.1 mol/m² hr and each type of catalyst may be soaked indifferent volumes of DI water, ranging between 40 mL to 500 mL. A seconddata set 904 shows reduction rates for each type of catalyst aftersoaking in DI water at 90° C. for 62 hours. Comparison of the first dataset 902 to the second data set 904 for each type of catalyst shows thatferric iron reduction rates increase by at least 100% for each type ofcatalyst after soaking, increasing by, for example, up to 3.8 times.Furthermore, greater volumes of DI water used to soak the catalyst bedmay be correlated with higher increases in reduction rate due to greateravailability of more water molecules to strengthen an overall van derWaals force compelling removal of chloride anions from catalystsurfaces.

Turning to FIG. 13 , it illustrates a plot 1300 of the percent recoveredperformance of a degraded catalyst bed versus a volume of DI water usedfor the DI soaking treatment as applied to various catalyst bed types,each of the data points representing DI soaking of a different catalystbed with equivalent catalyst surface area. Plot 1300 infers thatrecovery in performance of a degraded catalyst bed may be aided byconducting the DI soaking treatment with a ratio of DI watervolume/catalyst surface area that is above a threshold ratio. In oneexample, a threshold ratio of DI water volume to catalyst surface area,e.g., minimum volume of DI water to achieve a maximum performancerecovery, may be 10,000 mL/m², 15,000 mL/m², 20,000 mL/m² or greater forevery 0.01 m² of catalyst surface area.

A first treatment process for a catalyst bed of a rebalancing reactormay include soaking the catalyst bed in DI water at a target temperatureand a target period of time, as described above with respect to FIGS. 8and 9 . Soaking the rebalancing reactor may be an effective method tostrip adsorbed anions from a catalyst surface, allowing hydrogenmolecules to diffuse readily to reactive catalytic sites of the catalystbed and reducing a likelihood of double diffusion layer formation. Thefirst treatment may not, however, be conducted during operation of therebalancing reactor (and IFB) due to removal of the catalyst bed fromthe rebalancing reactor in order to submerge the catalyst bed in DIwater. As a result, application of the first treatment, as describedabove, may be delayed until a significant decrease in catalyticperformance is apparent.

In an alternative embodiment of the first treatment process, thecatalyst bed may be flushed with DI water instead of soaked. Whileflushing the catalyst bed may not occur simultaneously with activeoperation of the rebalancing reactor, flushing of the catalyst bed maybe conducted without removing the catalyst bed from the rebalancingreactor, thereby reducing an amount of time spent treating therebalancing reactors and reducing operational downtime thereof. Forexample, a redox flow battery, such as the redox flow battery 10 of FIG.1 with at least one rebalancing reactor, e.g., the rebalancing reactors80 and 82 of FIG. 1 , may be coupled to a DI water system. Water in theDI water system may be adapted to supply DI water from a reservoir toone or more of the rebalancing reactors 80 and 82, thereby flushing DIwater through the rebalancing reactor(s), including flushing thecatalyst bed therein with DI water. A portion of the DI water system,such as the reservoir, may be heated upstream of the rebalancing reactorto deliver DI water heated to a desired temperature (e.g., at atemperature above the threshold DI soaking temperature) to the catalystbed. Upon exiting the catalyst bed, the DI water containing ionsreleased from the catalyst bed, may be collected in a waste tank fordisposal or purification.

Flow of DI water from the DI water system to the catalyst bed may becontrolled by a valve positioned upstream of the rebalancing reactor andactuated by a controller, such as the controller 88 of FIG. 1 . The flowbattery system may be configured to block flow of electrolyte to therebalancing reactor, e.g., by closing a valve arranged in an electrolytechannel upstream of the rebalancing reactor, when the DI water systemvalve is commanded to open. The DI water system valve may be opened toallow water to flush the catalyst bed when a performance of therebalancing reactor is detected to be diminished. The valve may beopened for a set period time to deliver a target volume of water to thecatalyst bed based on a controlled flow rate of the DI water. The DIsystem may also include a valve positioned downstream of the rebalancingreactor.

In one example, the downstream valve may be closed while the upstreamvalve is opened to allow the rebalancing reactor to fill with DI water.The catalyst bed may soak in the DI water for a period of timedetermined to be sufficient to desorb ions from the catalyst bed. In oneexample, the period of time may correspond to a time greater than athreshold DI soaking time, as described above. The downstream valve maythen be opened to drain the DI water when the period of time elapses,followed by closing of the upstream DI water system valve and re-openingof the valve upstream of the rebalancing reactor that controlselectrolyte flow to the rebalancing reactor.

Alternatively, DI water may be continuously flushed through the catalystbed, until the DI water is sufficiently low in ion concentration to deemthe catalyst bed cleaned of adsorbed anions. The DI water emerging fromthe catalyst be may be monitored for a resistivity of the water,allowing flushing of the catalyst bed to proceed until the resistivityof the water decreases below a threshold DI soaking resistivity. In oneexample, the threshold DI soaking resistivity may be 18 MΩ·cm or less.Reducing the resistivity of the DI water emerging from the soakedcatalyst bed below the threshold DI soaking resistivity may aid inreducing anion adsorption at the catalyst surface. By adapting the firsttreatment process to be performed without removal of the catalyst bed,the catalyst bed may be more efficiently restored to a desiredperformance with decreased downtime, e.g., deactivation, of therebalancing reactor.

In some examples, the first treatment process may be configured eitheras a DI water soaking process that demands removal of a catalyst bedfrom a rebalancing reactor or as a purging process that flushes thecatalyst bed within the rebalancing reactor with DI water. In otherexamples, both configurations of the first treatment process may beapplied to the rebalancing system. The purging process may be used as arelatively quick and efficient method to periodically strip anions fromthe catalyst bed so that the rebalancing balance is deactivated for abrief period of time. The soaking process may be used as a lessfrequent, deep treatment of the catalyst bed to more thoroughly removeadsorbed anions from a catalyst surface.

Although configuring the first treatment process to be applied withoutremoving the catalyst bed from the rebalancing reactor may offer afaster option for maintaining catalyst performance, flushing of thecatalyst bed may nonetheless demand a deactivation of the rebalancingreactor and redox flow battery over a duration of the first treatmentprocess. Thus an additional treatment method that may be used in situduring operation of the redox flow battery, and more specifically, anIFB, may be desirable to more routinely reduce catalytic degradation inthe rebalancing reactor.

A second treatment process may also address adsorption of chlorideanions onto a catalyst surface. The second treatment process may includeapplying a negative potential to a catalyst bed of a rebalancing reactorduring operation of the rebalancing reactor. More specifically, thesecond treatment may be applied during charging of the IFB when hydrogengas is generated as a side reaction. Alternatively, the second treatmentmay be used whenever the IFB is operated, regardless of cycle. Thenegative potential may be applied to the catalyst bed, structured as ajelly roll, by conductively coupling a conductive wire to the catalystbed, as shown in FIGS. 2-3C. In one example, conductively coupling aconductive wire to the catalyst bed may include weaving a conductivewire, such as a titanium wire, inside the jelly roll (e.g, the catalystbed 300 of FIG. 3C) and extending out through a top of the jelly roll,and applying a negative potential to the conductive wire.

The conductive wire may extend throughout an entire axial length of thecatalyst bed, as shown in FIG. 3C, so that the negative potential isapplied across the axial length of the catalyst bed. Furthermore, theconductive wire may be wrapped and/or woven throughout successiveconcentric layers of the jelly roll structure such that the negativepotential is applied throughout each successive concentric layer of thejelly roll structure. In one example, the conductive wire may beconductively coupled and/or woven into one or more of the substratelayer 304, catalyst layer 306, and the spacing layer 310 prior to spiralwinding the aforementioned layers to form the jelly roll structure. Thismay facilitate distributing the conductive wire and the negativepotential more thoroughly across the catalyst bed volume, which can aidin increasing recovered performance of a degraded catalyst bed. Forexample, the conductive wire may be formed from titanium and woventhrough plastic mesh used as a substrate for preparing the jelly rollstructure. In a larger jelly roll structure, two titanium wires may bewoven through the plastic mesh along the entire axial length prior torolling the jelly roll structure with the catalyst bed. By weaving theconductive wire through the plastic mesh, good contact between thecatalyst and the conductive wire is enabled, allowing uniform chargingof the catalyst across a surface area of the catalyst bed.

A counter electrode formed from titanium mesh, carbon electrode orgraphite felt, may be placed in an electrolyte of the rebalancingreactor, the ratio of the counter electrode surface area to a surfacearea of a catalyst supported on the catalyst bed being less than athreshold surface area ratio. In one example, the threshold surface arearatio may be less than or equal to 0.2 (e.g., one fifth). Maintainingthe ratio of the counter electrode surface area to the catalyst bedsurface area less than or equal to 0.2 may aid in reducing a risk ofdegradation of the catalyst bed due to poisoning of catalyst sites byanions. The counter electrode may be placed in the electrolyte tank orin the electrolyte path. Since the negative potential is applied on thecatalyst surface to repel anions, no current flows through the counterelectrode.

An effect of an applied potential on catalyst performance is shown ingraph 1000 in FIG. 10 . Graph 1000 includes a rate of ferric ironreduction at a catalyst bed of a rebalancing reactor along the y-axis,measured in moles per square meter hour, and time elapsed since initialexposure to electrolyte along the x-axis, in hours. A first plot 1002shows a baseline trend line for a first catalyst bed in a firstrebalancing reactor operated without an applied potential to the firstcatalyst bed. A second plot 1004 shows a trend line for a secondcatalyst bed in a second rebalancing reactor operated with a negativevoltage above a lower threshold negative voltage applied to the secondcatalyst bed. In one example, the lower threshold negative voltage maybe −50 mV. In another example, the lower threshold negative voltage maybe −400 mV. Furthermore, the negative voltage applied to the catalystbed may be below an upper threshold negative voltage. Applying anegative voltage to the catalyst bed less than an upper thresholdvoltage may reduce a risk of generating enough current to inhibit therate of ferric ion reduction at the catalyst bed due to electrochemicaloxidation of ferrous to ferric ion at the catalyst surface. In oneexample, the upper threshold negative voltage may be −800 mV. The firstplot 1002 shows consistently lower reduction rates than the second plot1004 during a first interval 1004 over a duration of graph 1000.

The results shown in graph 1000 indicate that application of thenegative potential to the catalyst bed increases catalyst performanceduring application of the potential. However, when the applied potentialis removed, the reduction rate may drop rapidly to the baseline rate butmay be restored to a higher reduction rate when the negative potentialis re-applied to the catalyst bed.

The continuous application of the negative potential to the catalyst bedin the rebalancing reactor may impose a negative charge on the catalystbed. The negative charge may repel chloride anions, suppressing anionadsorption onto the catalyst surface and reducing a likelihood offormation of the charge double diffusion layer. During operation of therebalancing reactor the negative potential may be continuously appliedto the catalyst bed or selectively applied during charging of an IFBcoupled to the rebalancing reactor. A magnitude of the negativepotential may be varied when catalyst performance is determined to bedegraded. For example, if a ferric iron reduction rate is detected todecrease below a threshold rate, an external electrical device providinga voltage to the conductive wire woven into the catalyst bed may beincreased (e.g., a more negative potential applied) to increase anegative charge on the catalyst bed and to increase a repulsive forcediscouraging adsorption of anions at a surface of the catalyst bed.

The second treatment process may be routinely used during IFB operationand during rebalancing reactor operation, including continuouslyapplying the negative potential to the catalyst bed to retard catalyticdegradation resulting from adsorption of chloride anions onto thesurface of the catalyst. In contrast, the first treatment process may bea supplemental treatment, whereby DI soaking or purging of the catalystbed may be performed as an additional treatment during conditions whendegradation of the catalyst reaches beyond a threshold degradationlevel, despite the continuous negative overpotential. For example, thethreshold degradation level may be reached when a ferric iron reductionrate decays below a threshold reaction rate, indicating that therebalancing reactor performance is degraded. In another example, thethreshold degradation level may be indicated when an electrolyte pH isgreater than a first threshold pH. As such, a rebalancing reactorcatalyst bed may be degraded and hydrogen gas production, resulting fromiron metal corrosion and proton reduction, may occur at higher rateswhich may consume protons and increase the electrolyte pH. In anotherexample, the threshold degradation level may be reached when a batterycharge capacity is reduced; when a rebalancing reactor catalyst bed isdegraded, plating at the negative electrode of the IFB battery cell(s)may be reduced, thereby reducing battery charge capacity.

In one case, the second treatment may be used continuously when the IFBis in operation. Over numerous charging and recharging cycles, aperformance of the rebalancing reactor catalyst may decrease due toformation of the charge double diffusion layer at the catalyst surface,resulting in a decrease in the ferric iron reduction rate at therebalancing reactor. Upon detection of the decrease in the reductionrate, the first treatment process may be applied, e.g., the catalyst bedmay be flushed with DI water while the rebalancing reactor isdeactivated or the rebalancing reactor may be deactivated and thecatalyst bed may be removed and soaked in DI water.

In another example, both of the two embodiments of the first treatmentprocess may be used in combination with the second treatment process.For example, the second treatment may be applied continuously during IFBoperation, at routine intervals of operation of the IFB, or when theferric reduction rate drops below a first threshold rate to maintain theferric reduction rate above the threshold rate. When the secondtreatment is no longer able to maintain the ferric reduction rate abovethe threshold, a DI flush of the first treatment process may be used torestore catalytic performance. The DI soak of the first treatmentprocess may be used periodically when even the DI flush does notmaintain catalytic performance or may be used as treatment betweenlonger periods of operation, such as every 200 hours of IFB operation.

It will be appreciated that the examples described above arenon-limiting examples of how the first and second treatment processesmay be implemented. Various combinations of the methods associated withthe first and second treatment processes have been contemplated. Thenegative potential of the second method may be a relatively lowoverpotential of, for example, −400 mV vs. hydrogen potential, thusreadily supplied by a small battery such as a lithium ion or nickelmetal hydride battery. Replacement of the battery may contributeminimally to overall system costs. Both the first and second treatmentprocesses may be low-cost, simple and effective methods, eitherindependently or collaboratively, to increase catalytic performance inthe rebalancing reactor for an IFB. A first method 1100 for reducingperformance degradation of a catalyst bed of a rebalancing reactor bygenerating a surface charge (e.g., the second treatment processdescribed above) is shown in FIG. 11 .

The catalyst bed of the first method 1100 may be the jelly rollstructured catalyst bed 320 of FIGS. 3B-3C, configured as a stackincluding a layer of platinum, or another catalytic metal, supported oncarbon, a layer of a carbon substrate, and a spacing layer. The catalystbed, rolled into a coil, may be arranged in at least one rebalancingreactor coupled to a redox flow battery, with respect to the rebalancingreactors 80, 82 of the redox flow battery 10 of FIG. 1 . The redox flowbattery may be an all-iron flow battery (IFB), adapted with electrolytecomprising dissolved ferric and ferrous iron complexes and supportingnon-redox active salts, such as KCl. The IFB may have a controller, suchas the controller 88 of FIG. 1 , receiving information from sensors ofthe IFB and rebalancing reactors, such as temperature sensors in thepositive and negative electrolyte chambers, and sending instructions toactuators of the IFB, such as valves controlling flow of electrolyte.The controller may refer to instructions stored on a memory of thecontroller to execute commands based on data received from the sensors.A pH of the electrolyte may be acidic to maintain a stability of theelectrolyte. Hydrogen gas, a by-product at a negative electrode of theIFB, may be generated and channeled to the rebalancing reactor to beoxidized by the catalyst bed to protons. Ferric iron may also be reducedto ferrous iron at the carbon substrate of the catalyst bed.

At 1102, the method includes operating the IFB. For example, pumps maybe activated to pump electrolyte through a cell of the IFB, the cellincluding positive and negative electrode compartments. The IFB mayundergo either charging, implementing equations (1) and (2) in a forwarddirection, or discharging, during which equations (1) and (2) occur in areverse direction. When the IFB is actively charging, side reactionssuch as proton reduction and iron corrosion, according to equations (3)and (4), may result in a rise in a pH of the electrolyte and a loss ofelectrolyte stability.

At 1104, the method includes applying a negative potential to thecatalyst bed. The negative potential, which may be in a range between−400 to −600 mV, may be applied to the catalyst bed of the rebalancingreactor by a conductive wire woven into the catalyst bed, such as atitanium wire, coupled to an electrical device that supplies electricalpower. For example, the electrical device may be a lithium ion batteryor a nickel metal hydride battery. The battery may be connected to theconductive wire by a switch upon determination that the electrolyte pHsurpasses the first threshold. Alternatively, the battery may becontinuously coupled to the wire and the negative potential applied tothe catalyst bed constantly, regardless of IFB operating mode or activeflow of electrolyte through the rebalancing reactor. A counter electrodemay be arranged in the electrolyte. Application of the overpotential tothe catalyst bed may impose a negative charge on the catalyst bed,repelling anions in the electrolyte that may otherwise adsorb to thecatalyst surface.

The method proceeds to 1106 to determine if the pH rises above a firstthreshold. The first threshold may be a pH, such as pH 4, above which alikelihood of iron oxide formation increases, where precipitation ofiron oxide may lead to loss of iron cations available for redoxactivity. The pH may be measured by a pH meter that sends electrolyte pHinformation to the controller. If the pH is not above the firstthreshold, the method proceeds to 1108 to continue operating the IFBunder current conditions, such as in the charging or discharging mode.The method then returns to the start. However, if the pH is measured tobe above the first threshold, the method proceeds to 1110.

At 1110, the method includes decreasing the pH of the electrolyte.Decreasing the electrolyte pH includes flowing electrolyte from the IFBto the rebalancing reactor at 1112. A valve arranged in a line betweenan electrolyte storage tank of the IFB and the rebalancing reactor maybe opened to allow electrolyte to flow from the electrolyte storage tankto the rebalancing reactor. Adjusting the electrolyte pH also includeschanneling hydrogen gas from the electrolyte storage tanks to therebalancing reactor at 1114. Hydrogen gas produced by equations (3) and(4) may accumulate in the electrolyte storage tanks and be siphoned tothe rebalancing reactor. As hydrogen gas and electrolyte flows into therebalancing reactor, the hydrogen gas diffuses to the catalyst surfaceand may be oxidized to generate protons. Simultaneously, ferric iron maybe reduced to ferrous ion. The electrolyte, containing elevatedconcentrations of protons and ferrous ion, is recirculated to the cellof the IFB at 1116 to rebalance the pH and iron speciation of theelectrolyte in the cell.

At 1118, the method determines if the ferric iron reduction rate in therebalancing reactor is above a second threshold. The ferric ironreduction rate may be measured by cyclic voltammetry, redox titration,or an oxygen reduction potential meter. The second threshold may be arate of reduction that is sufficiently slow to indicate that catalystperformance is degraded to a degree where electrolyte instability and pHrise is imminent. For example, the second threshold may be set at 0.6mol/m² hr. If the reduction rate is determined to not fall below thefirst threshold, the method returns to 1104 to evaluate whether theelectrolyte pH is rising above the first threshold.

If the iron reduction rate is determined to decay below the secondthreshold, the method proceeds to 1120 to flush the catalyst bed with DIwater flow. Flowing DI water to the rebalancing reactor to flush thecatalyst bed may include commanding a first valve arranged upstream ofthe rebalancing reactor in an electrolyte channel to close. The firstvalve in the electrolyte channel may be positioned between the IFB celland the rebalancing reactor, controlling flow of electrolyte andhydrogen gas from the IFB cell to the rebalancing reactor. Upon closingthe first valve in the electrolyte channel, a second valve in the DIwater system may be commanded to open. The second valve may be arrangedin a DI water channel flowing DI water from a heated reservoir of DIwater to the rebalancing reactor. In some examples, the DI water systemmay include a third valve arranged downstream of the rebalancingreactor. The third valve may be instructed to close during the DI waterflush to retain DI water in the catalyst bed to allow the catalyst bedto soak in a volume of DI before the DI water is purged from therebalancing reactor. Alternatively, the third valve may be commanded toopen during flow of DI water through the rebalancing reactor.

As the catalyst bed is exposed to DI water, anions adsorbed onto thecatalyst surface may be stripped off and flushed out of the rebalancingreactor. At 1122, the method determines if a resistivity of the DI waterexiting the rebalancing reactor is at a resistivity equal to or greaterthan a third threshold. The resistivity of the DI water after flushingthe catalyst bed may be measured by, for example, a resistivity probe inthe DI water system, to infer an ionic concentration in the water. Thethird threshold may be a resistivity that indicates that the waterleaving the rebalancing reactor is no longer stripping ions from thecatalyst bed due to a complete removal of ions from the catalyst bed.The third threshold may be a resistivity similar to that of pure DIwater, such as 18 MΩ·cm. If the resistivity of the exiting water is notat the third threshold, the method returns to 1120 to continue flowingDI water from the reservoir through the rebalancing reactor.

If the resistivity of the water reaches or surpasses the thirdthreshold, the method proceeds to 1124 to close the second valve of theDI water system, halting flow of DI water to the rebalancing reactor.The first valve is instructed to open, directing electrolyte from theIFB cell to the rebalancing reactor. As charging of the IFB isconducted, hydrogen gas generated at the negative electrode may besiphoned to the rebalancing reactor to be oxidized. The electrolyte iscirculated from the rebalancing reactor to the battery cell aftertreatment at the catalyst bed. Method 1100 then continues to method 1200of FIG. 12 .

In other examples, the third threshold may instead be a target intervalof time or predetermined volume of water rather than the resistivity ofthe outflowing water. For example, the third valve may be closed,allowing the rebalancing reactor to fill with DI water. When therebalancing reactor is filled, a timer may be activated, set at a periodof time estimated sufficient to remove anions from the catalyst surface.Alternatively, the catalyst bed may be flushed with a volume of watercalculated to be a suitable volume of water to flush anions off thecatalyst surface and restore a desired performance of the catalyst.

At 1202 of method 1200, the method includes determining if an operationinterval, e.g., number of hours of operation, of the IFB has reached afourth threshold. The fourth threshold may be a duration of time overwhich the IFB has been consistently in operation, such as 100 hours or200 hours. The interval of time defined by the fourth threshold may be aperiod of elapsed operation time calculated or estimated to lead to anelevated likelihood of double diffusion formation at the catalystsurface and a decrease in performance of the rebalancing reactor. If theoperation interval has not reached the fourth threshold, the methodproceeds to 1204 to continue operation of the IFB, e.g., charge mode ordischarge mode, while circulating electrolyte through the rebalancingreactor.

If the operation interval reaches the fourth threshold, the methodcontinues to 1206 to halt flow of electrolyte and hydrogen gas to therebalancing reactor. Blocking flow of electrolyte and hydrogen gas tothe rebalancing reactor may be achieved by closing the first valve ofthe electrolyte channel. The method may proceed to 1206 regardless ofwhether indications of performance degradation are received, e.g.,whether or not the first through third thresholds are exceeded. Arequest for treatment of the catalyst bed by soaking in DI water isindicated by the controller at 1208. The request may be indicated byactivating an alert signal, such as a light or an alarm, to inform anoperator that treatment of the catalyst bed is demanded. The request forsoaking the catalyst bed may be a remedial treatment or an anticipatorytreatment, depending on a status of the catalyst. Soaking the catalystbed may include removal of the catalyst bed from the rebalancing reactorand submerging the catalyst bed in a known volume of water at a settemperature over a target period of time.

When the operator reinstalls the catalyst bed and indicates that therebalancing reactor is ready for operation, e.g., by entering a commandin a communication device of the controller or by pressing a button todeactivate the alarm, etc., the method proceeds to 1210 to resumecirculating electrolyte and siphoning hydrogen gas to the rebalancingreactor. The method then returns to the start of method 1100 of FIG. 11.

The soaking of the catalyst bed in DI, at a target temperature,duration, and volume, may afford a more thorough removal of anions fromthe catalyst surface than the DI flushing process described above. Whenthe catalyst bed is removed from the rebalancing reactor and soakedexternally, the catalyst bed may be unrolled, thereby allowing a greatersurface area of the catalyst bed to be in direct contact with a largervolume of DI water than flushing the catalyst bed while still coiled inthe rebalancing reactor. Furthermore, the catalyst bed may be moreefficiently and uniformly exposed to water heated to a highertemperature in an oven, when the catalyst bed is removed and unrolled,than may be achieved by heating the reservoir of DI water in the DIwater system coupled to the rebalancing reactor prior to delivering theDI water to the rebalancing reactor. Thus the DI soaking process may bea more effective method to thoroughly remove anions from the catalystsurface with a caveat of demanding more time and effort to conduct. Itmay therefore be desirable to apply the DI soaking process lessfrequently than the DI flushing of the rebalancing reactor.

In this way, a pH of a flow battery electrolyte may be maintained,thereby allowing the electrolyte of the flow battery to be balanced,with regards to state of charge and concentrations of redox activespecies, for a prolonged period of redox flow battery operation. Theredox flow battery may be an all-iron flow battery (IFB) relying on ironredox reactions to induce electron flow through battery. Competing sidereactions at a negative electrode of the IFB during charging may resultin consumption of protons, driving a rise in electrolyte pH and leadingto electrolyte imbalance. Coupling the IFB to a rebalancing reactorwhere the generating hydrogen may be oxidized back to form protons whilereducing ferric iron to ferrous iron may restore the electrolytebalance. However, a catalyst of the rebalancing reactor may be prone toadsorption of anions, such as chloride, derived from supporting salts inthe electrolyte, and leading to formation of a double diffusion layeraround catalyst sites. The double diffusion layer may impede interactionbetween hydrogen gas molecules and the catalyst and degrade catalystperformance in a manner that leads to progressive degradation overprolonged battery usage. The formation of the double diffusion layer maybe circumvented by a applying a constant negative potential to thecatalyst bed, e.g., the second treatment process described above. Thenegative potential imposes a negative charge on the catalyst bed,thereby repelling anions from the catalyst surface and suppressingformation of the double diffusion layer. Over time, however, ions maynonetheless become adsorbed onto the catalyst surface. Deionized watermay be used to remove the double diffusion layer by exposing thecatalyst to heated deionized (DI) water. The catalyst bed may either beflushed with DI water while housed in the rebalancing reactor or removedfrom a housing of the rebalancing reactor and soaked in DI water.Exposing the catalyst bed to DI water may strip away anions from thecatalyst surface, thus reestablishing catalyst activity. Both applying anegative potential to the catalyst bed and removing anions via DI watermay be cost efficient, simple methods to maintain and/or increasecatalyst performance, thereby providing a continuous, in situ treatmentprotocol in combination with a more thorough, periodic treatmentroutine.

The technical effect of treating a rebalancing reactor with the firsttreatment process of soaking the catalyst bed in DI water in betweenusage of the IFB and/or the second treatment process of applying aconstant negative potential to the catalyst bed during operation of theIFB is that the pH of the electrolyte is maintained, prolonging a usefullifetime of the IFB.

In one embodiment, a method includes flowing an electrolyte of the flowbattery and hydrogen gas generated in the flow battery to therebalancing reactor, applying a negative potential to a catalyst bed ofthe rebalancing reactor while flowing the electrolyte, detecting adecrease in a ferric iron reduction rate at the catalyst bed below athreshold rate, flowing deionized water instead of electrolyte acrossthe catalyst bed in response to the decrease in the ferric ironreduction rate; and indicating, after a threshold interval ofrebalancing reactor operating time elapses, a request for soaking of thecatalyst bed in deionized water. In a first example of the method,applying the negative potential to the catalyst bed includes coupling aconductive wire to the catalyst bed, and transmitting a voltage from anelectric device to the catalyst bed. A second example of the methodoptionally includes the first example, and further includes, whereinapplying the negative potential to the catalyst bed includes generatinga negative charge on the catalyst bed, the negative charge repellingelectrolyte anions from the catalyst bed, and maintaining the negativepotential above a threshold potential during operation of therebalancing reactor. A third example of the method optionally includesone or more of the first and second examples, and further includes,wherein generating the negative charge on the catalyst bed includesmaintaining the negative charge on the catalyst bed while the redox flowbattery is charging A fourth example of the method optionally includesone or more of the first through third examples, and further includes,wherein flowing the deionized water across the catalyst bed includeshalting flow of electrolyte and hydrogen gas to the rebalancing reactorand wherein halting flow of electrolyte to the rebalancing reactorincludes closing a first set of valves controlling flow between thebattery cell and the rebalancing reactor. A fifth example of the methodoptionally includes one or more of the first through fourth examples,and further includes, wherein flowing deionized water across thecatalyst bed includes opening a second set of valves controlling flowbetween a deionized water reservoir and the rebalancing reactor, thedeionized water reservoir fluidly coupled to the rebalancing reactor. Asixth example of the method optionally includes one or more of the firstthrough fifth examples, and further includes, wherein soaking thecatalyst bed includes halting flow of electrolyte to the rebalancingreactor by closing the first set of valves, removing the catalyst bedfrom the rebalancing reactor, and submerging the catalyst bed in heateddeionized water for a predetermined period of time in a predeterminedvolume of deionized water. A seventh example of the method optionallyincludes one or more of the first through sixth examples, and furtherincludes, wherein flowing deionized water across the catalyst bedflushes the catalyst bed and wherein the flushing terminates when thedeionized water emerging from the rebalancing reactor reaches a targetresistivity.

In another embodiment, a system includes, an electrolyte circulatingthrough a cell of the redox flow battery and hydrogen gas stored inelectrolyte chambers of the redox flow battery, both the electrolyte andhydrogen gas flowed to a rebalancing reactor coupled to the cell, anegative potential applied to the catalyst bed during charging of theredox flow battery, a controller, configured with computer readableinstructions stored on non-transitory memory, the instructionsexecutable by the controller to apply a negative potential to thecatalyst bed during charging of the redox flow battery, flush thecatalyst bed with deionized water upon detection of the rate of ferriciron reduction at the rebalancing reactor falling below a secondthreshold, and indicate a request for soaking of the catalyst bed indeionized water when an interval of operating time of the redox flowbattery is elapsed. In a first example of the system, the electrolyteincludes ferric and ferrous iron complexes, non-redox active salts, andan acid. A second example of the system optionally includes the firstexample, and further includes, where the hydrogen gas is generated at anegative electrode of the cell in a process that consumes protons fromthe electrolyte. A third example of the system optionally includes oneor more of the first and second examples, and further includes, whereinthe catalyst bed of the rebalancing reactor has a jelly roll structure.A fourth example of the system optionally includes one or more of thefirst through third examples, and further includes, wherein the negativepotential applied to the catalyst bed is configured to generate anegative charge at the catalyst bed and wherein the negative potentialis applied continuously during operation of the redox flow battery. Afifth example of the system optionally includes one or more of the firstthrough fourth examples, and further includes, wherein the catalyst bedis flushed with deionized water while housed in the rebalancing reactorwhen deionized water is flowed through the rebalancing reactor. A sixthexample of the system optionally includes one or more of the firstthrough fifth examples, and further includes, wherein the catalyst issoaked when the redox flow battery system is deactivated and thecatalyst is removed from the rebalancing reactor.

In yet another embodiment, a method includes flowing an electrolyte ofthe flow battery and hydrogen gas generated in the flow battery to therebalancing reactor, detecting a decrease in an iron reduction rate ofthe rebalancing reactor, and responsive to the decrease in ironreduction rate, halting flow of electrolyte and hydrogen gas to therebalancing reactor and flowing deionized water through the rebalancingreactor. In a first example of the method, detecting the decrease in theiron reduction rate of the rebalancing reactor includes measuring theiron reduction rate to be below a threshold rate that decreases aperformance of the rebalancing reactor. A second example of the methodoptionally includes the first example, and further includes, whereinflowing deionized water includes directing deionized water from areservoir to the rebalancing reactor. A third example of the methodoptionally includes one or more of the first and second examples, andfurther includes maintaining a negative charge on a catalyst bed of therebalancing reactor during operation of the flow battery. A fourthexample of the method optionally includes one or more of the firstthrough third examples, and further includes, wherein flowing theelectrolyte to the rebalancing reactor includes delivering theelectrolyte from a battery cell of the redox flow battery to therebalancing reactor and wherein the rebalancing reactor is configured torestore a pH and ferrous iron concentration of the electrolyte.

In another representation, a method for treating a rebalancing reactorof a redox flow battery includes flowing an electrolyte of the redoxflow battery and hydrogen gas generated in the flow battery to therebalancing reactor, and applying a negative potential to a catalyst bedof the rebalancing reactor while flowing the electrolyte and hydrogengas to the rebalancing reactor. In a first example of the method,flowing the electrolyte to the rebalancing reactor includes deliveringthe electrolyte from a battery cell of the redox flow battery to therebalancing reactor to restore a pH and ferrous iron concentration ofthe electrolyte. A second example of the method optionally includes thefirst example, and further includes, wherein applying the negativepotential to the catalyst bed includes continuously maintaining thenegative potential to repel anions. A third example of the methodoptionally includes one or more of the first and second examples, andfurther includes, wherein applying the negative potential to thecatalyst bed includes activating an electric device electrically coupledto the catalyst bed by a conductive wire. A fourth example of the methodoptionally includes one or more of the first through third examples, andfurther includes, wherein applying the negative potential to thecatalyst bed includes impeding formation of a double diffusion layer atthe catalyst bed and increasing a ferrous iron reduction rate.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

The invention claimed is:
 1. A redox flow battery system, comprising; anelectrolyte circulating through a cell of the redox flow battery andhydrogen gas stored in electrolyte chambers of the redox flow battery,both the electrolyte and hydrogen gas flowed to a rebalancing reactorcoupled to the cell; a negative potential applied to the catalyst bedduring charging of the redox flow battery; a controller, configured withcomputer readable instructions stored on non-transitory memory, theinstructions executable by the controller to: apply a negative potentialto the catalyst bed during charging of the redox flow battery; flush thecatalyst bed with deionized water upon detection of the rate of ferriciron reduction at the rebalancing reactor falling below a secondthreshold; and indicate a request for soaking of the catalyst bed indeionized water when an interval of operating time of the redox flowbattery is elapsed.
 2. The redox flow battery system claim 1, whereinthe electrolyte includes ferric and ferrous iron complexes, non-redoxactive salts, and an acid.
 3. The redox flow battery system of claim 1,where the hydrogen gas is generated at a negative electrode of the cellin a process that consumes protons from the electrolyte.
 4. The redoxflow battery system of claim 1, wherein the catalyst bed of therebalancing reactor has a jelly roll structure.
 5. The redox flowbattery system of claim 1, wherein the negative potential applied to thecatalyst bed is configured to generate a negative charge at the catalystbed and wherein the negative potential is applied continuously duringoperation of the redox flow battery.
 6. The redox flow battery system ofclaim 1, wherein the catalyst bed is flushed with deionized water whilehoused in the rebalancing reactor when deionized water is flowed throughthe rebalancing reactor.
 7. The redox flow battery system of claim 1,wherein the catalyst is soaked when the redox flow battery system isdeactivated and the catalyst is removed from the rebalancing reactor.